1. Field of the Invention
The present invention relates to a grain oriented silicon steel sheet used for the iron core of a transformer or a generator. The grain oriented electromagnetic steel sheet has a high magnetic flux density and improved iron loss properties, and is particularly suitable for enabling downsizing of a transformer. The invention further relates to a new method of manufacturing such grain oriented sheet, and to the slab from which it is made.
2. Description of the Related Art
A grain oriented electromagnetic steel sheet containing silicon and having crystal grains oriented in a (110)[001] or (100)[001] orientation has excellent soft magnetic properties. Such sheets widely serve as various iron core materials in the commercial frequency range. An important property that a grain oriented steel sheet is required to have in such uses is a low iron loss. The iron loss is usually evaluated as power loss upon magnetization at a frequency of 50 Hz to 1.7 T (hereinafter expressed as W17/50 (W/kg).
Regarding the iron loss property of a commercial large-capacity transformer, those having a stacked iron core or a coiled iron core, constructed using a grain oriented electromagnetic steel sheet having low W17/50 iron loss values are quite excellent. Further, a grain oriented electromagnetic steel sheet having a high magnetic flux density with an improved orientation of crystal grains is beneficial for the purpose of downsizing a large-capacity transformer. The consumption of such materials is increasing year by year, partly urged by the tendency toward energy saving.
In a grain oriented electromagnetic steel sheet, only crystal grains of a particular orientation are selectively caused to grow. This is done by utilization of a phenomenon known as secondary recrystallization.
In the usual process of manufacturing a grain oriented electromagnetic steel sheet, it is necessary to cause fine precipitation of an inhibitor in the steel. This is done by hot rolling after solid-solution treatment of the inhibitor, by heating to a high temperature a steel slab containing the inhibitor component in the steel. Simultaneous use of AlN and MnS as inhibitors, and simultaneous use of AlN and MnSe, are commonly employed for increasing the high magnetic flux density.
The inhibiting function of AlN is readily influenced by the secondary recrystallization annealing atmosphere. As a result, the magnetic properties of the sheet tend to become unstable.
There is disclosed a bismuth-containing grain oriented electromagnetic steel sheet with a view to achieving a high magnetic flux density. Japanese Unexamined Patent Publication No. 56-18044 discloses a method of manufacturing a grain oriented electromagnetic steel sheet, using MnS and MnSe as inhibitors, wherein bismuth is added to a steel slab and pre-rolling of the slab is finished at a temperature of up to 1,050xc2x0 C.
Japanese Examined Patent Publication No. 56-21331 discloses a technique based on a combination of bismuth, AlN and MnS and a combination of bismuth, AlN and MnSe.
Japanese Examined Patent Publication No. 7-62176 discloses, as Example 3, a technique of annealing, for a minute, a hot-rolled steel sheet containing aluminum, sulfur and bismuth at 1,000xc2x0 C. by use of a two-stage cold rolling, subjecting the resulting steel sheet to intermediate annealing at 1,050xc2x0 C., rapidly cooling the same, and applying an aging treatment.
The technique disclosed in the aforementioned Japanese Unexamined Patent Publication No. 56-18044 provided, however, only an ineffective value of magnetic flux density.
In the techniques of the aforementioned Japanese Examined Patent Publication No. 56-21331 and Japanese Examined Patent Publication No. 7-62176, with the use of AlN as an inhibitor, the secondary recrystallization annealing atmosphere sometimes caused fluctuations of magnetic properties, resulting in an unstable iron loss value entirely unsuitable for industrial manufacturing.
A germanium-containing grain oriented electromagnetic steel sheet is disclosed as a technique for obtaining a low iron loss. Japanese Unexamined Patent Publication No. 59-31823 discloses a technique for obtaining a satisfactory value of W17/50 by enriching the slab inner layer with germanium.
Japanese Unexamined Patent Publication No. 2-196403 discloses a technique for obtaining a satisfactory W17/50 value based on a combination of germanium and AlN, or a combination of germanium, AlN and MnS, or a combination of germanium, AlN and MnSe.
In the technique disclosed in the aforementioned Japanese Unexamined Patent Publication No. 59-31823, however, it is essential to enrich the slab inner layer with germanium, making it industrially difficult to add wires upon slab casting. Reducing the size of secondary recrystallization grains is also unavailable.
In the technique disclosed in the aforementioned Japanese Unexamined Patent Publication No. 2-196403, on the other hand, it is essential to use AlN as an inhibitor. This may sometimes cause fluctuations of magnetic properties due to the effect of the atmosphere upon secondary recrystallization annealing, resulting in an unstable iron loss value. This technique is not acceptable for industrial application.
Under such circumstances, we carried out extensive studies of manufacturing techniques using an inhibitor other than MnS, MnSe or AlN. This resulted in development of a manufacturing technique using boron nitride as an inhibitor for making a grain oriented electromagnetic steel sheet having a high magnetic flux density. An application for patent was filed (Japanese Examined Patent Application No. 8-301474).
The use of BN as an inhibitor has been disclosed. For example, Japanese Examined Patent Publication No. 58-43445 discloses a technique using a steel containing from 0.0006 to 0.0080 wt % boron and 0.0100 wt % nitrogen. However, the grain oriented electromagnetic steel sheet so obtained has a magnetic flux density B8 of only about 1.89 T at most, along with only a fair iron loss. The technique previously developed by the present inventors, in contrast, is based on a method using a combination of BN and MnS or BN and MnSe as an inhibitor, and changing the hot rolling conditions in response to the silicon content and the amount of added boron. According to this technique, it is possible to stably obtain a grain oriented electromagnetic steel sheet having a very high magnetic flux density. However, the demand for improvement of magnetic properties is still increasing for transformers and the like using grain oriented electromagnetic steel sheets from the point of view of product downsizing and energy saving. The grain oriented electromagnetic steel sheet serving as a material for iron cores is therefore required to have even a still higher magnetic flux density and a further reduced iron loss. Furthermore, in a BN-containing grain oriented electromagnetic steel sheet having a high magnetic flux density, crystal grains of the product tend to be coarser. In some cases, therefore, the iron loss value was not necessarily comparable to the magnetic flux density value. There has been room for improvement regarding achievement of a lower iron loss.
The present invention has therefore an object to provide a grain oriented electromagnetic steel sheet using BN as an inhibitor and having a further reduced iron loss and a high magnetic flux density.
We have discovered a new way to manufacture an electromagnetic steel sheet having a low iron loss and a high magnetic flux density. This is done by adding an element to the steel which accelerates not only precipitation of fine inhibitive BN in the steel but also achieves beneficial precipitation of silicon nitride during the manufacturing process, and radically improves the texture of the primary recrystallized grains of the steel sheet immediately before subjecting the same to secondary recrystallization annealing. This invention further combines a texture-improving treatment with primary recrystallization annealing and cold rolling.
More specifically, addition of bismuth or germanium into the steel, or both, with the application of appropriate primary recrystallization annealing conditions is effective as a step in the process.
Further, such a combination with addition of germanium into the steel, and application of appropriate primary recrystallization annealing conditions with warm rolling, is particularly effective.
In addition to the above-mentioned findings, we have found how to achieve acceleration of beneficial precipitation of silicon nitride by critically limiting the contents of harmful impurities, particularly aluminum and vanadium.
The present invention provides a method of manufacturing a grain oriented electromagnetic steel sheet having a high magnetic flux density and a very low iron loss, comprising the steps of reheating a steel slab containing from about 0.030 to 0.095 wt % carbon, from about 1.5 to 7.0 wt % silicon, from about 0.03 to 2.50 wt % manganese, from about 0.003 to 0.040 wt % sulfur and/or selenium, and from 0.0010 to 0.0070 wt % boron at a temperature of over 1,350xc2x0 C., then hot-rolling the reheated steel slab, subjecting the resulting hot-rolled steel sheet to one or more stages of cold rolling under conditions including a final cold rolling of from about 80 to 95% into a final thickness, conducting primary recrystallization annealing, then coating an annealing separator on the sheet, and applying final annealing, wherein Bi or Ge is added, such element improving fine BN precipitation and improving the texture of primary recrystallized grains of the steel sheet immediately before secondary recrystallization annealing; and wherein N is added in an amount of from 30 to 120 wtppm to the steel slab to precipitate silicon nitride; the aluminum content is controlled to about 0.015 wt % or less and the vanadium content is controlled to about 0.010 wt % or less, as impurities. The important hot rolling conditions include a hot rolling time within a range of from about 50 to 220 seconds, a hot rolling finishing temperature of at least about 850xc2x0 C., rapid cooling at a cooling rate of at least about 30xc2x0 C./sec upon completion of hot rolling, and coiling at a temperature of up to about 700xc2x0 C. Appropriate primary recrystallization conditions and warm rolling are combined to improve the texture.
Bismuth is added in an amount of from about 0.0005 to 0.100 wt %. This has been discovered to accelerate precipitation of fine BN, having a fineness of about 10-500 nm in average diameter in the decarburized sheet, improving the texture of primary recrystallized grains of the steel sheet immediately before subjecting the steel sheet to secondary recrystallization annealing; and primary recrystallization under conditions appropriate for improving the texture, including a heating rate of at least 8xc2x0 C./sec at a temperature of at least 500xc2x0 C. in the primary recrystallization annealing, and an annealing temperature of from 800 to 900xc2x0 C.
We have further provided a method of manufacturing a grain oriented electromagnetic steel sheet having a high magnetic flux density and a very low iron loss, wherein germanium is added in an amount of from about 0.005 to 0.500 wt % as an element accelerating precipitation of fine BN and improving the texture of primary recrystallized grains of the steel sheet immediately before subjecting the steel sheet to a secondary recrystallization annealing. Primary recrystallization conditions appropriate for improving the texture of the material include a heating rate of at least about 5xc2x0 C./sec at a temperature of at least about 500xc2x0 C. in the heating step of the first annealing during cold rolling, and an annealing temperature of from about 1,000 to 1,150xc2x0 C.; the final cold rolling comprises a warm rolling at a maximum temperature within a range of from about 150 to 350xc2x0 C.
It is desirable also to utilize the addition of a trace element to assist the inhibitor, or a nitriding treatment during the period after decarburization annealing and before the secondary recrystallization. It is also desirable to practice magnetic domain refining, or formation of a tensile film on the steel surface at an appropriate stage.
The final product of the invention is a grain oriented electromagnetic steel sheet having a high magnetic flux density and a very low iron loss, comprising up to about 0.010 wt % carbon, from about 1.5 to 7.0 wt % silicon, from about 0.03 to 2.50 wt % manganese, up to about 0.003 wt % sulfur and/or selenium, from about 0.0004 to 0.0030 wt % boron, and up to about 30 wtppm nitrogen, wherein aluminum is limited to about 0.002 wt % or less, and vanadium is limited to about 0.010 wt % or less, as impurities. An element (Bi or Ge or both) is added for accelerating fine precipitation of BN, thereby improving the texture of primary recrystallized grains of the steel sheet immediately before subjecting the sheet to secondary recrystallization annealing.
A final product of the invention is a grain oriented electromagnetic steel sheet that contains from about 0.005 to 0.100 wt % bismuth and/or from about 0.005 to 0.500 wt % germanium. This accelerates fine precipitation of BN, and improves the texture of primary recrystallized grains of the steel sheet immediately before subjecting the steel sheet to secondary recrystallization annealing.